The present invention relates to metal honeycombs useful as fluid heaters or heated catalyst supports for automotive emissions control catalysts. More particularly the invention relates to improved electrically conductive metal honeycombs for the fabrication of such devices, and to methods for making them.
One approach to meeting the impending low and ultra-low automotive exhaust emission control standards for pollution control is the use of a fast-starting catalytic conversion unit such as an electrically heated catalyst (EHC). Electrically heated catalysts, which generally employ an electrically activated heater at a location supporting or otherwise proximate to a conventional emissions control catalyst, can be of various designs.
One recently developed design, based on an electrically conductive metal honeycomb made by the extrusion and sintering of selected metal alloy powders, has proven particularly effective for this application. Examples of extruded metal honeycombs for this application are disclosed, for example, in U.S. Pat. No. 5,194,719 to Merkel et al.
The manufacturing methods used to make extruded metal honeycombs impose a number of structural limitations on the product. For example, to achieve the uniform extrusion of metal powder batches of extrudable consistency through honeycomb extrusion dies of conventional design, the cell walls, outer skin, and other structural parts of the honeycomb have had to be of roughly the same wall thickness. This is because variations in wall thickness tend to be accompanied by variations in extrusion rate, the latter typically leading to undesirable distortions in the cell structure of the resulting extruded green honeycomb bodies.
These limitations have made it difficult to extrude metal honeycombs in forms which lend themselves to the subsequent application, in a mechanically rugged configuration, of contact electrodes for the application of electrical power to the honeycomb. The thin honeycomb cell walls needed for high electrical resistivity and efficient heating do not provide sufficient supporting structure for the firm mounting of the electrodes.
Another difficulty with thin-walled metal honeycombs relates to the tendency of the peripheral wall structure to abrade or otherwise damage mechanical or electrical insulation material used to isolate and support the honeycombs in the exhaust system. This is particularly harmful if electrical insulation between the honeycomb and its grounded support or container is thereby damaged.
One prior art solution to these problems has involved a process step known as "edge fill". In this process a plasticized powdered metal filler material is used to fill in selected channels located at or near the edge of the extruded honeycomb shape. Upon consolidation of an edge-filled honeycomb the filled channels form an edge or skin which can be smoothed as necessary, and to which electrodes for powering the honeycomb can be successfully attached. However, while edge filling effectively addresses some of the problems relating to the use of thin-walled metal honeycombs for electrically heated catalyst use, it represents a time-consuming and expensive process.
In the manufacture of ceramic honeycomb bodies by extrusion, a number of different modifications to the extrusion dies have been proposed to help with the formation of desirable skin configurations on the extruded ware. These include U.S. Pat. No. 4,075,270 describing scalloped mask constructions, and U.S. Pat. No. 5,219,509 relating to die structures providing means for transitioning extrudate flow for the honeycomb skin to smoothly follow the direction of extrusion of the honeycomb core.
In the polymer extrusion art, methods and apparatus have been developed for co-extruding core and skin polymers of differing composition employing an extruder providing separate feed streams for the core and skin materials. However, as illustrated by U.S. Pat. No. 5,240,396, this approach requires a relatively complex die structure in order to maintain separate feed streams for the extruded materials.
Therefore, a method for increasing the outer skin or wall thickness of extruded metal honeycombs for use as electrical heaters without the need for supplemental edge-filling or other complex processing equipment for the honeycombs would be of significant economic benefit.